Comet encounter exceeds UW astronomer’s greatest hopes

In a surprise to scientists, the nucleus of comet Wild 2 turned out to be greatly textured with depressions and craters in images returned to Earth by the Stardust spacecraft. The image at left shows the pockmarked nucleus under natural lighting and the overexposed image at right, necessary for Stardust’s systems to track the comet, reveals several jets of material spewing into space.

After five long years, the Stardust spacecraft is on its way home with a trove of microscopic particles gleaned from comet Wild 2 in a spectacular encounter the day after New Year’s.

The flyby greatly exceeded expectations and now UW astronomy Professor Donald Brownlee, the primary scientist for the NASA mission, is eagerly waiting for the spacecraft to send its return capsule parachuting to Earth in January 2006 so particles can be parceled out to laboratories around the world.

But even before the capsule plops down in the Utah salt flats, the Stardust mission has already provided a wealth of data — including more than 70 photographs beamed back following the comet encounter.

“It will revolutionize what we know about comets, just from the pictures, which is something we didn’t expect,” Brownlee said Monday from Pasadena, Calif., where he and others involved with the mission continued to pore over data sent from Stardust.

Scientists expected that the number of particles streaming toward the spacecraft would increase as it got closer to the comet and then would decrease again as it pulled away, but that’s not what happened, he said.

“We saw a lot of impacts close in, but we also saw a lot of impacts at other distances because we flew through jets,” he said. “These jets are sonic-speed streams of gas shooting out into the vacuum.”

In fact, jets from the comet are distinctly visible in the closest-ever images of a comet that were sent back shortly after the encounter 242 million miles from Earth. The jets are made up of volatile substances that quickly turn to gas as they interact with the sun.

One unexpected aspect, Brownlee said, was to see large crater-like depressions, some more than a half-mile across, on the surface of the comet nucleus. Those depressions apparently were left when volatile substances jetted into space, though some also might have been caused by impacts.

Brownlee has worked toward a comet encounter for 25 years, and believes the success of Stardust “could prove to be a pivotal time for science, a remarkable opportunity to gather evidence that might actually tell us how the planets formed and give us clues about how life on Earth began.”

Stardust flew through the gas-and-dust envelope surrounding the nucleus of Wild 2, coming within about 150 miles of the core. Material streaming from the comet surface at nine times the speed of a rifle bullet rocked the spacecraft, but it survived without a hitch. A tennis-racquet shaped collector, using a wispy foam material called aerogel, gathered perhaps a thimbleful of microscopic comet dust, while a high-gain antenna beamed photographs and data to the mission’s nerve centers at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., and Lockheed Martin Space Systems in Denver.

Thousands of tons of comet particles blanket Earth each year but they are difficult to find among earthly materials, Brownlee said. “And even when extraterrestrial particles can be found, they are cosmic orphans — there is no way to determine their origin.”

Such particles also couldn’t give the same kind of information as those taken directly from an object like Wild 2. Before 1974, the comet orbited the sun in a path that extended from Jupiter to beyond Uranus. That year Jupiter’s gravity altered Wild 2’s course, bringing its orbit close enough — just beyond the orbit of Mars — to make the Stardust mission feasible. Because it only moved closer to the sun 30 years ago, there hasn’t been enough time for the sun’s heat to destroy characteristics of the comet that have been preserved in the cold of deep space for billions of years.

Stardust, the fourth in the National Aeronautics and Space Administration’s series of Discovery missions and the first mission designed to return samples from beyond Mars, was launched from Cape Canaveral, Fla., on Feb. 7, 1999. It is currently on its third giant loop around the sun, and will have traveled some 3.1 billion miles by the end of its voyage.

Stardust is a collaboration of the UW, NASA and its Jet Propulsion Laboratory at the California Institute of Technology, and Lockheed Martin Space Systems. Other key members are the Boeing Co., the Max-Planck Institute for Extraterrestrial Physics, NASA Ames Research Center and the University of Chicago.